Various prior art fixation systems are employed to arrest the relative motions and transfer the overturning moments and loads between the hull and legs of a jackup rig.
A jackup rig as used herein means any working platform for drilling, work over, production, crane work, compressor station, diving support or other offshore purpose in an elevated position above the water, and being supported on jackable legs to the ocean floor or other water bottom, with the inherent capability of relocating from one site to another by lowering to a floating position, and after being moved to a new established location, raising again to an elevated position. Jackup rigs are equipped with rack and pinion jacking systems for raising and lowering the platform. The rack is fixed to the leg along its vertical length, and pinions and their drives are disposed on the jacking units' structures which are connected to the hull.
The first type of fixation systems use stoppers or lock bolts which individually engage with and lock into the tooth spaces or on the tooth flanks of the rack on the leg. These stoppers can be moved individually or as group by actuators. However, the individual stopper is capable of adjusting its position when it meshes with the tooth spaces of the rack. Exemplary of the first type of fixation systems are U.S. Pat. Nos. 4,447,401, 4,813,814, and 5,139,366. Each of these systems requires manual intervention during engagement notwithstanding certain automation features. Where the movement of stoppers is in a direction substantially perpendicular to the leg rack, the stoppers may not engage fully into the tooth space in all relative positions of the hull with respect to the leg.
By rack chock fixation system, the present invention refers to a system for aligning and locking a section of rack chock with a profile matching the teeth of the rack on the jackable legs. U.S. Pat. No. Re. 32,589 exemplifies the rack chock fixation system where a series of vertical and horizontal screw jacks align a rack chock piece with flat upper, lower and lateral surfaces with a matching teeth profile on the leg rack. The alignment is accomplished manually and visually. Once aligned, the rack chock and the leg rack are locked by screw jacks.
Another type of locking fixation system for jackup rig is exemplified by U.S. Pat. No. 4,389,140 which teaches the use of a rack chock with inclined upper and lower surfaces with flange edges held between claws of vertically movable wedges with mating surface and matching inclination. The wedges are guided to move vertically by nuts fixed into the wedges; the rack piece is engaged with mating teeth on the leg rack by independent rotating screws. Rotation of the screws with identical speed in the same direction moves the wedges as well as the rack vertically. Rotation of the screws with identical speed in the opposite direction brings the wedges closer and moves the rack piece laterally by sliding along the inclined surfaces. Similarly the locking fixation system requires visual monitoring for the exact matching of the rack chock with the teeth of the leg rack.
The third type of fixation system for jackup rigs is typified in U.S. Pat. No. 4,662,787 which teaches the use of a rack chock with inclined upper and lower surfaces having flanged edges held between claws of horizontally movable wedges with mating surface and matching inclination. Wedge guides in the framework (attached rigidly to the hull) guided the wedges, while independent screwjacks acting against the flat end of the wedges provided movement to it. Rotation of the screwjacks in opposing directions with the same speed moves the rack chock vertically. Rotation of the screwjacks in the same direction moves the inclined surfaces of the wedges towards or away from each other, thus moving the rack chock toward or away from the leg rack by sliding along the inclined faces of the wedges.
All three types of prior art fixation system employ combinations of rack chocks, wedges, and jacks to arrest the relative movements between legs and hull as well as to transfer the loads in a self elevating platform. It can also be said that all prior art fixation systems require visual monitoring for the exact meshing of the teeth of the rack chock with the teeth of the leg rack.